The present invention concerns a method of synchronizing a reference clock of a ground station and a clock of a remote system, in particular a satellite, which can be less accurate than the reference clock.
A number of problems are associated with establishing an absolute time reference.
Although there is a universal time reference, it is not available in real time. This universal time reference is referred to as xe2x80x9ccoordinated universal timexe2x80x9d (UTC) and is generated by the co-operation of around 50 countries, each of which has an official time reference center with very accurate atomic clocks which provide their own estimate of the UTC time. Consequently, to obtain the exact value of the real UTC time, it is necessary to collect all the data and to perform a number of calculations, as a result of which the exact value of the UTC time is available at the end of a delay which is in the order of approximately one month. Thus there is no physical clock which maintains UTC time, which means that if such a clock is required in real time an approximation is needed.
The best available approximation of UTC time is GPS (Global Positioning System) time. This is kept synchronized with UTC time with an accuracy which in practice is in the order of 20 ns. However, this technique does not allow real time dissemination of GPS time, because of the selective availability of the GPS signal and because of noise on the link during communication of the GPS frequency and time. It is necessary to average the GPS signals over a period of approximately one day to acquire GPS time with the nominal accuracy, in particular for synchronizing communications networks. This requires a relatively good clock which is stable over a period of approximately one day.
Transferring time information T and frequency information F from one point to another introduces errors which limit the accuracy and stability of the recovered clock. The Navigation Satellite Broadcast (NSB) system is based on the best possible estimate of the propagation time D when transmitting data between the ground and space. A clock CK(t) transmitted at a time t is received as a clock CK(t+D) at a time t+D and any uncertainty as to the value of D is reflected in a phase error for the received clock and thus inaccuracy in respect of the time T and the frequency F.
However, most current systems operate in an open loop configuration, with no interaction or feedback between the received clock and the reference clock.
One prior art bidirectional satellite communications technique does not evaluate the delay D directly but effects a time comparison by exchanging data between the two clocks, on the assumption that the uplink and downlink delays D are equal. This concept does not use a closed loop. That technique is described in U.S. Pat. No. 4,792,963 (CAMPANELLA). It uses a phase correction which is accumulated during a sidereal day and is then applied for the next sidereal day. However, although that method provides an effective correction that prevents long-term drift, it is not suitable for correction in real time.
The present invention proposes to reduce the discrepancy between the quality of highly accurate clocks of fixed installations on the ground and the performance of methods for transmitting this precise and stable time together with frequency information.
It relates to a new method of transferring time and frequency information via satellite in real time which is based in particular on direct measurement of the delay D and which is limited only by system noise.
One object of the invention is therefore to synchronize a local clock of a remote installation, such as a satellite, with a reference clock of a ground station.
Another object of the invention is to synchronize two or more ground stations with each other by iterative use of the method.
Another object of the invention is to synchronize the local clock of another satellite when the clock of a first satellite has been synchronized with the ground station including the reference clock.
Another object of the invention is to improve the accuracy over that obtained with prior art techniques by a factor of up to two orders of magnitude.
Another object of the invention is to obtain improved accuracy in real time and in a manner which is compatible with universal coverage.
At least one of the above objects is achieved by a method of synchronizing a reference clock of a first ground station and a local clock of a first remote system, in particular a moving body and notably a satellite, the reference clock signals being contained in streams of reference pulses referred to hereinafter as bursts (or reference salvoes) transmitted by the ground station and received by the first remote system, said bursts beginning with a recognition word, the method including:
a) acquisition in a first loop of preliminary synchronization between said reference bursts received by the first remote system and bursts generated by the first remote system and synchronized with the local clock,
b) detection of the recognition word of the synchronized reference bursts received by the first remote system and generation after detection of said recognition word of a time window containing a given number N of pulses of said reference clock,
c) acquisition of the average phase of said pulses received from the reference clock in the time window and comparison with the phase of the local clock, and
d) in a second loop, synchronization of the phase of the local clock with said average phase to achieve said synchronization of the local clock.
The method can further include:
e) final synchronization of the reference bursts (salvoes) which are compensated by the propagation delay D of the uplink signal and said bursts generated by the remote system.
In the method, the acquisition of synchronization can entail:
a1) in the first remote system, comparing the time of reception of the recognition words of the reference bursts transmitted by the ground station and the start of the bursts generated by the first remote system, and
a2) in the ground station, adjusting the transmission of the reference bursts so that said reception time coincides with said start of said generated bursts.
The method can in particular use a time-division multiple access (TDMA) multiplex uplink to transmit the reference bursts between the ground station and the moving body.
The method can also use a time-division multiplex (TDM) downlink between the first remote system and the ground station.
To synchronize a second remote system, namely a second satellite and/or a second ground station, after said synchronization of the onboard clock of the first remote system, the method can iterate said synchronization such that the reference clock is said synchronized local clock of the first remote system and the local clock to be synchronized is a local clock of a second remote system, said iteration including:
axe2x80x2) acquisition in a third loop of preliminary synchronization between bursts transmitted by the first remote system, which constitute reference bursts beginning with a recognition word, and bursts generated by the second remote system and synchronized with the local clock of the second remote system,
bxe2x80x2) detection of the recognition word of the bursts transmitted by the first remote system which are received by the second remote system and synchronized and generation after detection of said recognition word of a time window including a given number Nxe2x80x2 of pulses received from said synchronized local clock,
cxe2x80x2) acquisition of the average phase of said pulses received from said synchronized local clock and comparison with the phase of the local clock, and
dxe2x80x2) in a fourth loop, synchronizing the phase of said local clock of the second remote system with said average phase to synchronize the local clock of the second remote system.
The method can also include:
exe2x80x2) final synchronization of said reference bursts (salvoes) and said bursts generated by the second remote system.
The method can be such that:
axe2x80x21) in the second remote system, it compares the time of reception of the recognition words of the synchronized bursts transmitted by the first remote system and the start of the bursts generated by the second remote system, and
axe2x80x22) in the first remote system, it adjusts the transmission of said transmitted bursts so that said time they are received by the second remote system coincides with the start of said generated bursts.
Potential applications of the method according to the invention include:
generating in space a universal time reference based on a high-performance atomic clock in a ground station, without it being necessary to install a high-performance clock on the satellite,
precisely comparing time and frequency with more accurate clocks on the ground, for applications in the field of industrial calibration or requiring real time clocks (the accuracy that can be achieved can be two orders of magnitude greater than that currently available using the GPS),
precisely synchronizing digital communications networks and exchanges,
navigation and positioning in the vicinity of the Earth and in space, and
comparing clocks on the ground a great distance apart.